1. Field of the Invention
The present invention relates to a discharge tube, a light source employing the discharge tube, and a method of producing the discharge tube.
2. Description of Related Art
A discharge tube such as a metal vapor discharge tube is conventionally used for a light source employed in a liquid crystal (LC) projection device in which the light source irradiates an LC panel with light to thereby project an LC imaging light onto a screen. The discharge tube is generally of a double-tube structure in which an inner bulb is sealingly enclosed in the interior of an outer tube. The inner bulb is formed with an arc discharge chamber in which a pair of electrodes are sealingly enclosed to be opposed to each other. The pair of electrodes serve to generate arc discharge therebetween in the arc discharge chamber so as to generate light. The inner bulb is sealingly enclosed in a cylindrically-shaped outer tube. The interior of the outer tube is in a vacuum state or is filled with a small amount of inert gas so as to thermally insulate the inner bulb enclosed in the outer tube from atmospheric air outside, to thereby enhance the heat accumulating capacity of the inner bulb. The discharge tube of the double-tube structure therefore has the following great advantage relative to a discharge tube of a single-bulb structure in which the inner bulb is not enclosed in the outer tube but is exposed to atmospheric air outside. That is, the volume of the inner bulb (i.e., the volume of the arc discharge chamber of the inner bulb) of the double-tube structure type attainable of a desired light emission efficiency with a rated electrical power is larger than that of the single-bulb structure type attainable of the desired light emission efficiency with the same rated electrical power. In other words, in the discharge tube of the double-tube structure, it is possible to attain the desired light emission efficiency with the rated electrical power, with the inner bulb of a larger volume, relative to the discharge tube of the single-bulb structure. Accordingly, the total area of the inner surface of the inner bulb of the double-tube structure becomes larger than that of the single-bulb structure. In the case where the discharge tubes of the double-tube structure and of the single-bulb structure are operated for the same period of time, the same amount of matter is spattered on the inner surfaces of the inner tubes. Accordingly, even in the case where the discharge tubes of the double-tube structure and of the single-bulb structure are operated for the same period of time, the amount of the matter spattered on an inner surface of the inner bulb of the double-tube structure per unit area becomes smaller than that of the matter spattered on an inner surface of the inner bulb of the single-bulb structure per unit area. Therefore, the lifetime of the discharge tube of the double-tube structure becomes larger than that of the single-bulb structure.
In order to produce the above-described discharge tube of the double-tube structure, the inner bulb is inserted into the interior of the outer tube from one of a pair of end portions of the outer tube, and the end portions of the outer tube is sealed.
Japanese Unexamined Patent Application Publication Nos. 61-78044 and 3-37951 disclose the discharge tubes of the double-tube structure type. In the discharge tube disclosed in the publication No.61-78044, an air suction pipe is sealingly passed through a sealing portion which is formed at one end of an outer tube thereof through pressure deformation treatment so that one end of the air suction pipe is positioned in the interior of the outer tube. A lead wire which is connected, at its one end, to an inner bulb is tied, at its other end, on the end of the air suction pipe positioned inside of the outer tube. Thus, the inner bulb is held in the interior of the outer tube in such a manner that the inner bulb is supported via the lead wire on the air suction pipe.
In the discharge tube disclosed in the publication No.3-37951, a tip end of the air suction pipe is projected inwardly of the outer tube from one end thereof. An inner bulb is supported via a mounting member on the tip end of the air suction pipe so that the inner bulb is held in the interior of the outer tube.
Each of the above-described discharge tubes of the publication Nos.61-78044 and 3-37951 has, however, such problems that the inner bulb supporting mechanism thereof is intricate in its structure and therefore has a low strength.
In order to solve the above-described problem, a discharge tube as shown in FIG. 1 has been proposed. The discharge tube 72 shown in FIG. 1 is of the double-tube structure in which an inner bulb 76 is sealingly enclosed in a cylindrically-shaped outer tube 77. The inner bulb 76 has a pair of columnar-shaped solid portions 80 for sealingly supporting therein a pair of electrodes 74 and a spherically-shaped hollow portion 75 defining a spherically-shaped arc discharge chamber 75' for enclosing therein xenon, metal vapor, or the like. The spherically-shaped hollow portion 75 is positioned between the solid portions 80 so that the pair of electrodes 74 may be projected from the solid portions into the arc discharge chamber 75'. The pair of electrodes 74 serve to generate an arc discharge therebetween in the arc discharge chamber 75'. A pair of lead wires 81 are connected to the pair of electrodes 74 embedded in the solid portions 80. The inner bulb 76 is held inside of the outer tube 77 in such a manner that the pair of lead wires 81 thus connected to the electrodes 74 are sealingly supported by a pair of sealed portions 82 which are air-shieldingly formed at both ends of the cylindrically-shaped outer tube 77. Thus, the inner bulb 76 is supported in the interior of the outer tube 77 via the pair of lead wires 81 in such a manner that the inner bulb 76 is not directly contacted with the outer tube 77. The interior of the outer tube 77 is in a vacuum state or is filled with a small amount of inert gas, so that the internal bulb 76 is thermally insulated from atmospheric air.
The above-described discharge tube 72 is simple in its structure so that it is possible to easily decrease the size of the discharge tube 72. The strength of the discharge tube 72 can be enhanced.
When the above-proposed discharge tube 72 is to be produced, the inner bulb 76 is first inserted into the cylindrically-shaped outer tube 77 through one of its opposed open ends. Then, the outer tube 77 is thermally deformed, at the open ends, into the sealed portions 82 so that the pair of lead wires 81 may be sealingly passed through the sealed portions 82, respectively. As a result, the inner bulb 76 is supported in the outer tube 77 via the pair of lead wires 81.
The above-described discharge tube 72 has been proposed to be attached to a reflective mirror so that a light source may be produced, as shown in FIG. 2.
FIG. 2 illustrates a light source to be employed for an LC projection device. The LC projection device includes the light source and an LC panel, and is positioned relative to a wide screen in such a position that the light radiated from the light source may pass through the LC panel to be projected onto the wide screen. In the LC projection device, the light source radiates light onto an LC panel which displays a desired image thereon so that a desired imaging light may be projected from the LC panel onto the wide screen.
As shown in FIG. 2, the light source 71 is constructed by attaching the discharge tube 72 as shown in FIG. 1 to a parabolic reflective mirror 73. The discharge tube 72 is inserted, at its mounting portion 78, into an access through-hole 79 of the reflective mirror 73, and is fixed thereto. The discharge tube 72 is attached to the parabolic reflective mirror 73 at such a position that a center position of the arc discharge chamber 75' may be positioned at a focal point F of the parabolic mirror 73. Light generated in the arc discharge chamber 75' proceeds in a forward direction of the reflective mirror 73 indicated by an arrow in FIG. 2 (leftward direction in FIG. 2) or proceeds in a rearward direction toward the surface of the reflective mirror 73 (rightward direction in FIG. 2) to be reflected thereat and proceed forwardly. The light source 71 therefore serves to radiate a parallel light beam in the forward direction (leftward direction in FIG. 2) so that the light beam may be effectively projected onto a surface of the LC display which is positioned forwardly of the light source 71.
It is noted that since it is necessary to insert the inner bulb 76 into the outer tube 77 through the open end of the cylindrically-shaped outer tube 77, the outer diameter of the outer tube 77 of the discharge tube 72 is larger than the outer diameter of the spherically-shaped hollow portion 75 of the inner bulb 76 as described above. Accordingly, the inner diameter of the access through-hole 79 of the reflective mirror 73 which is equal to or slightly larger than the outer diameter of the outer tube 77 is larger than the outer diameter of the hollow portion 75. The diameter of the access through-hole 79 is therefore large relative to the inner diameter of the arc discharge chamber 75'. Since the area of the access through-hole 79 confronting the arc discharge chamber 75' is thus large, a large part of the light beam radiated rearwardly from the arc discharge chamber 75' reaches the access hole 79 and fails to be reflected at the mirror surface 73. The light source 71 therefore has a problem that it fails to effectively or fully direct the light beam emitted from the discharge tube 72 forwardly to the LC display panel.
FIG. 3 illustrates another conventional discharge tube of the double-tube structure type which is proposed in "Designing with Metal Halide Lamps" (pp. 59-68 of "ELECTROOPTICAL SYSTEMS DESIGN" published in March of 1981). The discharge tube 172 is also in the double-tube structure including an inner bulb 176 and an outer tube 177 sealingly enclosing therein the inner bulb 176. The inner bulb 176 has a pair of columnar-shaped solid portions 180. In each of the solid-portions 180, an electrode 174, a pair of metal (molybdenum) foils 181 and 181' connected to one another are sealingly embedded in such a manner that the electrode 174 and the metal foil 181' may be projected from opposite ends of the each solid portion 180. The inner bulb 176 further has a substantially spherically-shaped hollow portion 175 at a position between the pair of columnar-shaped solid portions 180. The spherically-shaped hollow portion 175 defines therein an arc discharge chamber 175' for enclosing therein metal halide vapor gas and for generating arc discharge between the electrodes 174 and 174 which are projected in the arc discharge chamber 175' from the solid portions 180. The outer diameter of the spherically-shaped hollow portion 175 is larger than that of the columnar-shaped solid portions 180.
The outer tube 177 of the discharge tube 172 consists of a hollow portion 182 for receiving therein the spherically-shaped hollow portion 175 of the inner bulb 176. The outer tube 177 and the inner bulb 176 are continuously connected to each other in such a manner that the wall 182" of the hollow portion 182 of the outer tube 177 is connected to the pair of columnar-shaped solid portions 180 of the inner bulb 176.
Accordingly, the present inventors perceive that the discharge tube 172 may be combined with the parabolic reflective mirror 73 as shown in FIG. 2 in such a manner that the columnar-shaped solid portion 180 projected outwardly of the discharge tube 172 is inserted into the access through-hole 21 of the parabolic reflective mirror 73 and is fixed thereto. Since the diameter of the columnar-shaped solid portion 180 is smaller than that of the spherically-shaped hollow portion 175, it is possible to combine the discharge tube 172 with such a reflective mirror 73 as having the access through-hole 21 of a small diameter. In the case where the discharge tube 172 is thus fixed to the reflective mirror with the access through-hole of the small diameter, since the area of the access hole is small, only a small part of the light emitted rearwardly from the arc discharge chamber 175' reaches the access through-hole not to be reflected at the mirror surface. Accordingly, the obtained light source can effectively direct the light beam emitted from the discharge tube 172 forwardly.
The discharge tube 172 has, however, a problem that since the wall 182" of the outer tube 177 is in direct contact with the columnar-shaped solid portions 180 of the inner bulb 176, it is impossible to thermally insulate the inner bulb 176 from the atmospheric outside air reliably and certainly. In other words, the discharge tube 172 has the double-tube structure only at the arc discharge chamber 175' of the inner bulb 176, but has a single-tube structure at the columnar-shaped solid portions 180 of the inner bulb. With such a structure, it is impossible to fully or completely thermally insulate the inner bulb 176 from the atmospheric air. Accordingly, the life time of the discharge tube 172 is not so large with respect to that of the conventional discharge tube 72.